1. Field of the Invention
The invention relates to an over-voltage protection means which includes a first electrode and a second electrode having an air breakdown spark gap present between the electrodes, and further including a housing to holds the electrodes. When the air breakdown spark in the gap is initiated, an arc forms between the first electrode and the second electrode.
2. Description of Related Art
Electrical systems, including electronic measurement, control and switching circuits, as well as telecommunications means and systems, are sensitive to transient over-voltages that can occur due to atmospheric discharges, switching operations, or short circuits in the power supply grids. This sensitivity to over-voltage has increased to a degree in which electronic components, particularly when transistors and thyristors are used, are greatly endangered by transient over-voltages.
Electrical circuits operate normally without problems at the voltage specified for them, i.e., the rated voltage. Normal operation does not happen however when over-voltages occur. Over-voltages are considered to be all voltages which are above the upper tolerance limit of the rated voltages. They include mainly transient over-voltages which can occur not only from atmospheric discharges, but also from switching operations or short circuits in power supply grids. Such over-voltages can be conductively, inductively or capacitively coupled into electrical circuits. In order to protect electrical or electronic circuits, especially electronic measurement, control and switching circuits, and especially telecommunications means and systems, against transient over-voltages, over-voltage protection means have been developed and in use for more than twenty years.
An important component of an over-voltage protection means of the type which are the subject of the present invention is at least one spark gap which responds at a certain over-voltage, i.e., the sparkover voltage, and thus prevents over-voltages which are larger than the sparkover voltage of the spark gap from occurring in the circuit protected by the over-voltage protection means.
It was stated initially that the over-voltage protection means of the invention has two electrodes and an air breakdown spark gap which is present or which acts between the electrodes. In addition to over-voltage protection means with an air breakdown spark gap, there are over-voltage protection means with an air flashover spark gap in which upon activation a creeping discharge occurs. Over-voltage protection means with an air breakdown spark gap compared to a over-voltage protection means with an air flashover spark gap have the advantage of a greater current carrying capacity, but also have the disadvantage of a higher and not particularly constant sparkover voltage. Therefore different over-voltage protection means with an air breakdown spark gap have been proposed which have been improved with respect to the sparkover voltage. In the area of the electrodes, i.e., the air breakdown spark gap which acts between the electrodes, initiation aids have been developed. For example, between the electrodes there is an initiation aid made of plastic which triggers a creeping discharge by projecting like a crosspiece at least partially into the air breakdown spark gap.
An over-voltage protection means of this described type is disclosed in German Patent Document No. 44 02 615 C2. This over-voltage protection means has two, angular narrow electrodes in which each electrode has an arcing horn and a connecting leg angled therefrom. In addition, the arcing horns of the electrodes, in the area bordering the connecting legs, are provided with a hole. The holes provided in the arcing horns of the electrodes provide the capability, at the instant of initiation of the over-voltage protection element, for the resulting arc to be xe2x80x9cstartedxe2x80x9d by a thermal pressure effect which migrates away from its origin. Since the arcing horns of the electrodes are arranged in a V-shape to one another, the gap to be bridged by the arc is thus enlarged as the arc migrates away, thereby increasing the arc voltage.
When the air breakdown spark gap is initiated, the arc which forms causes a low-impedance connection between the two electrodes. For this reason, at the prevailing line voltage, an unwanted line follow current follows via the over-voltage protection means; therefore, an effort should be made to extinguish the arc as quickly as possible after the completed discharge process. One possibility for achieving this is to increase the arc length and thus the arc voltage. This solution is disclosed in the over-voltage protection means of the German Patent Document No. 44 02 615 C2. The disadvantage of this solution is that the geometrical dimensions of the electrodes become correspondingly large, and thus this solution is dependent on geometric considerations.
Another solution for extinguishing the arc is to cool the over-voltage protection means by the cooling action of insulation walls and by using insulators which release gas. In this solution a strong flow of the extinguishing gas is necessary which requires a major construction effort.
The object of this invention is to devise an over-voltage protection means of the type which is characterized by a high line follow current extinguishing capability, but which nevertheless can be implemented with a simple construction.
The over-voltage protection means of the invention in which the aforementioned object is achieved is characterized by a third electrode, along with the first electrode and the second electrode, in which between the first electrode and the third electrode a second air breakdown spark gap is present. The third electrode is connected via at least one impedance, e.g., a varistor, directly or indirectly to the second electrode, so that after discharging the surge current via the first electrode, the first air breakdown spark gap and the second electrode, the remaining arc can be moved from the first air breakdown spark gap to the second air breakdown spark gap, particularly by a pneumatic or magnetic blow-out.
As in the prior art, the over-voltage protection means of the invention is generally parallel to the input of the circuit, the system, or the device to be protected, and is conductively connected to the lines or terminals between which the operating voltage is applied in operation. As is normal, the first line or the first terminal is energized, while the second line or the second terminal is grounded. Using this terminology then, it is generally assumed that the first electrode of the over-voltage means should be connected to the energized line or terminal and the second electrode of the over-voltage protection means is connected to ground. Of course, the connections of the over-voltage protection means of the invention can be reversed, and the over-voltage protection means of the invention can be used not only to protect circuits in which there is an AC voltage as the operating voltage, but can also be easily used when the operating voltage of the circuit to be protected is a DC voltage.
As mentioned above, the over-voltage protection means includes a third electrode which is connected directly or indirectly to the second electrode via at least one impedance. A direct connection means that the third electrode is connected to the second electrode. An indirect connection of the third electrode to the second electrode means that this connection is made outside the over-voltage protection means, for example, the over-voltage protection means is a three-pole means in which both the second electrode and also the third electrode are grounded.
In the over-voltage protection means of the invention, the air breakdown spark gap initiates when the sparkover voltage is present, and as is conventional in the prior art, between the first and the second electrode. Additionally, to improve the response characteristic of the over-voltage protection means of the invention a known initiation aid can be implemented in the area of the air breakdown spark gap which acts between the electrodes such that for the initiated spark gap the surge current is discharged as is known. To suppress a possible line follow current or to extinguish a line follow current which may occur, the remaining arc is moved from the first air breakdown spark gap to the second air breakdown spark gap. Because the third electrode, unlike the second electrode, is not connected directly, but via at least one impedance, such as a varistor, for example, to ground, a suddenly increased impedance takes effect, so that a line follow current is prevented or an existing line follow current is extinguished. Due to the impedance connected downstream of the third electrode, between the first electrode and the energized line, the energized terminal and the ground, there is a voltage divider which provides for a partial voltage prevailing between the first electrode and the third electrode which is less than the arc voltage at the prevailing line voltage with the result that the partial voltage is thus no longer sufficient to maintain the arc.
The manner in which the arc remaining after discharge of the surge current is moved from the first air breakdown spark gap to the second air breakdown spark gap or from the first electrode and the second electrode to the first electrode and the third electrode, can be accomplished by different measures, for example, by pneumatic or magnetic blow-out. Pneumatic blow-out can be accomplished by the gas or plasma stream resulting from the arc thermal currents being guided in a controlled manner. One preferred embodiment of the over-voltage protection means of the invention which implements the pneumatic blow-out includes providing the housing and/or the third electrode with at least one opening, such that pressure equalization occurs through the opening and the pressure equalization causes a controlled propagation of the gas or plasma stream from the second electrode to the third electrode. With the propagation of the gas or plasma stream from the second electrode in the direction to the third electrode, the base of the arc is moved from the second electrode to the third electrode.
A magnetic blow-out can be accomplished by arranging the electrical terminals of the over-voltage protection means in the conventional manner such that the surge current produces a magnetic field which moves the arc from the first air breakdown spark gap to the second air breakdown spark gap, i.e., from the first electrode and second electrode to the first electrode and third electrode.
The over-voltage protection means of the invention can therefore include both pneumatic and also magnetic blow-out of the remaining arc.
This invention is fundamentally independent of the specific construction of the over-voltage protection means, particularly the type and shape of electrodes, the embodiment of the air breakdown spark gap, or the use of initiation aids. Two preferred embodiments of the over-voltage projections of the invention are briefly discussed below.
In the first preferred embodiment of the over-voltage protection means of the invention, the housing has an essentially cylindrical shape and the first electrode is made as a rod-shaped central electrode, while the second electrode and third electrodes are made as cylindrical outside electrodes which are arranged concentrically around the first electrode. The second electrode and third electrodes are located in a spaced axial relationship to one another such that part of the first electrode is surrounded by the second electrode and another part of the first electrode is surrounded by the third electrode. In this embodiment, the arc is then blown parallel to the lengthwise extension of the first electrode from the second electrode to the third electrode, such as by at least one radial opening located in the third electrode or at the transition of the third electrode to the housing.
A second preferred embodiment of the over-voltage protection means of the invention is characterized by the first electrode being made as a flat round disk, while the second electrode and third electrodes are located opposite the first electrode such that the second electrode is located centrally to the first electrode and the third electrode is located concentrically around the second electrode. This configuration of the electrodes yields an over-voltage protection means with a very short overall height. Preferably, the third electrode is not otherwise made in the shape of an annulus, but instead in the shape of a segment of an annulus, i.e., hemispherically, so that the third electrode concentrically surrounds the second electrode only partially. Furthermore, it is advantageous in one particular embodiment of the invention to make the axial distance between the first electrode and the second electrode smaller than the axial distance between the first electrode and the third electrode. This can be accomplished by different overall heights of the electrodes, or by arrangement of the second electrode and the third electrode. Because the distance between the first electrode and the second electrode is less than the distance between the first electrode and the third electrode, it is ensured that first the air breakdown spark gap initiates between the first electrode and the second electrode and the surge current is discharged via this air breakdown spark gap, and therefore via the first electrode and the second electrode.